System for combined cycle mechanical drive in cryogenic liquefaction processes

ABSTRACT

A system for producing liquefied and sub-cooled natural gas by means of a refrigeration assembly using a single phase gaseous refrigerant comprises at least two expanders; a compressor assembly; a heat exchanger assembly for heat absorption from natural gas; and a heat rejection assembly, in which the expanders are arranged in expander loops and the refrigerant to the respective expander is served in a compressed flow by means of the compressor assembly having compressors or compressor stages enabling adapted inlet and outlet pressures for the respective expander. According to the present the expanders and compressors assembly are assembled in two mechanically connected compressor and expander packages of which one is driven by a gas turbine and the other is driven by a steam turbine, the steam primarily being generated by exhaust gases from the gas turbine in a waste heat recovery unit, and in that the expanders and compressors assemblies are distributed between the two compressor and expander packages to optimize the steam utilization and to balance the power generated by the gas turbine and the steam turbine.

BACKGROUND OF THE INVENTION

Different variations of the Brayton cooling cycle have been continuously developed and described in patent literature over the years. The nature of the first single expander systems could speak in favour of utilizing the process for cooling applications, rather than for liquefaction, due to the fact that the refrigerant remains in single phase throughout the cycle and therefore is easier adaptable to single phase loads like gas cooling, for instance.

However, even though the traditional Brayton process did not perform as well in terms of liquefaction power as cascade processes, mixed refrigerant processes and other large scale liquefaction systems, the simplicity, robustness and flexibility of the process resulted in some popularity also for liquefaction purposes, e.g. for the production of liquefied natural gas (LNG). Especially for smaller plant the simple Brayton systems have proved to be feasible.

When regarded for use in larger scale natural gas liquefaction facilities, most factors mentioned above have been considered to be positive side effects of the Brayton loop, but the specific energy demand has overshadowed the other favourable effects compared to other technologies. Recent developments of the Brayton cycle like those described in Norwegian Patent Application 2008 3740, for instance, has narrowed the gap between the Brayton cycles and other technologies also in terms of energy performance. In addition to new requirements for more generic designs, safer processes and other aspects, these have contributed to revitalize variants of the Brayton loop for new medium to large scale LNG applications, e.g. in the floating LNG market.

A characteristic for most of the different process variants described in Norwegian Patent Application 2008 3740 is the large number of compression and expansion stages required. At first glance, this seems like a complicating factor when utilizing the processes for an application wherein equipment weight and dimensions need to be minimized. As shown hereinafter, this is not the fact.

Main objects of present invention are to provide for a solution establishing compact mechanical layouts, a low specific power demand, and a unique and optimal balancing of power consumers and suppliers for such processes. Another aspect is how to combine such mechanical layout in a cogeneration plant by direct mechanical drives for the compressor and/or expander units.

Thus, the system is utilising the energy balance in a dual or trippel Brayton cycle as to be optimized and flexible, as well as energy efficient. The system also facilitates a power balance that is dynamic between the different loops, i.e. low temperature, medium temperature and high temperature loop, involving the process lay out can handle fluctuations and varying conditions.

SUMMARY OF THE INVENTION

According to the present invention these object are achieved by a system for producing liquefied and sub-cooled natural gas by means of a refrigeration assembly using a single phase gaseous refrigerant comprising at least two expanders; a compressor assembly; a heat exchanger assembly for heat absorption from natural gas; and a heat rejection assembly, in which the expanders are arranged in expander loops and the refrigerant to the respective expander is served in a compressed flow by means of the compressor assembly having compressors or compressor stages enabling adapted inlet and outlet pressures for the respective expander, wherein the expanders and compressors or their stages are assembled in two mechanically connected compressor and expander packages of which one is driven by a gas turbine and the other is driven by a steam turbine, the steam primarily being generated by exhaust gases from the gas turbine in a waste heat recovery unit, and in that the expanders and compressors or their stages are distributed between the two compressor and expander packages to optimize the steam utilization and to balance the power generated by the gas turbine and the steam turbine.

Further embodiments of the present invention are specified in the dependent patent claims and the detailed description below.

Briefly, the present invention is involving a development of the double or triple Brayton loops as depicted by Norwegian Patent Application 2008 3740. The ambition is higher efficiency, i.e. less demand for kW per kg of the LNG produced, being an essential factor in today's operation to liquefy natural gas. The result of addressing this issue is a huge leap in the single refrigerant systems. The invention is unique in the respect that a gas turbine is providing the net power demand for a setup of at least one compressor and possibly at least one expander unit, and the exhaust gas from the gas turbine is used to generate steam. The steam is then routed to a steam turbine, driving another setup of at least one compressor and possibly at least one expander unit.

The low temperature or sub-cooling loop is physically placed on the gas turbine driven unit and the high temperature or de-superheating loop is preferably physically situated on the steam turbine driven unit. The medium temperature or condensation and cooling loop is typically split between the two. Since the level of generated power between the steam turbine and the gas turbine is in order of magnitude 2:1, this system layout is well suited for a process solution having dual or triple Brayton loops. This results in a setup with extremely low fuel demand compared to normal gas turbine driven systems without waste heat recovery units, and is hence economical, environmentally friendly and has low operating cost.

Via either a mechanical device such as a clutch, gear or other means, the system can be power balanced to fulfil the required power consumption of the total system, i.e. if any gas composition is such that thermodynamical alterations are insufficient to fulfil the power requirements, this can be done via a mechanical transfer instead. Otherwise this is normally achieved by process adjustments such as pressure levels, or via guide vanes in the compressor or expanders.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic lay out of a triple Brayton cycle as described in Norwegian Patent Application 2008 3740; and

FIG. 2 is a schematic lay out of the compressor and expander driver setup based on the triple Brayton cycle in FIG. 1 including a gas and steam turbine.

DETAILED DESCRIPTION OF THE INVENTION

As illustrated in FIG. 1, the present refrigeration assembly comprises three different refrigeration loops:

-   -   a high temperature loop, here depicted with two compressors 12,         13 or compressor stages and one expander 1 or expander stage,         hereinafter only denoted compressors and expanders,         respectively. As for all three loops this is a base case, but if         the process advantages are obvious, this lay out may consist of         a different number of compressor or expander units, depending         process requirements or other criteria. The high temperature         loop is used for de-superheating;     -   a medium temperature loop is used to condense the LNG, and this         loop normally consists of two compressors 14, 15 and one         expander 2; and     -   a low temperature loop consists of three compressors 16, 17, 18         and one expander in the base case also, and is used to sub-cool         the LNG.

Further information for the refrigeration assembly is to be found in the Norwegian Patent Application 2008 3740 and the same publication is incorporated here by reference for all relevant purposes.

Steam turbines based on waste heat recovery from gas turbines are well known in the process and power generation industries. In most cases at least one; cf. U.S. Pat. No. 6,640,586—often both—of the turbines in such a combined power generation unit are used for electrical power generation as the drivers are directly connected to an electrical generator.

The present invention describes a system where the two drivers of a combined cycle system in the form of a gas and steam turbine are used as direct mechanical drivers for different rotating machinery, i.e. compressors, expanders, gears, etc.

As already mentioned, the processes described by the Norwegian Patent Application 2008 3740 comprise at least two expander loops and an undefined number of compressors or compressor stages. Depending on the preferred process layout, especially in cases with one dedicated closed Brayton cycle for each expander loop, the number of compression and/or expansion stages can be quite high, see FIG. 1. This can be solved by employing a multiple number of drivers, but often a more compact design is mandatory due to weight and footprint limitations.

Integrally geared machines offers the potential of combining several compression and expansion stages on one common gearbox 205, 305, involving energy supply and consumption from the expanders and compressors, respectively. The number of stages capable of being integrated on one bullgear, i.e. one large toothed gear 206, 306 which in turn drives compressor or expander units attached via pinion gears 207, 307, are limited by the physical geometry of the impellers or volutes and size of the gear itself. Integrally geared machines normally combine two impellers for each pinion shaft rotating at the same speed. For normal integrally geared machines the number of pinion axes is limited to three, maximum four, which consequently allows for a maximum of six to eight impellers. Sometimes and depending on the drive speed the drivers themselves can also be connected to a pinion axis instead of the bull gear itself. This fact limits the maximum number of impellers per bull gear further compared to the number specified above.

In the process illustrated by FIG. 1 a total number of ten impellers is required, i.e. in any case more than capable of being connected to one single bullgear. In this case at least one further bullgear is likely to be introduced. FIG. 2 shows one mechanical setup of the process depicted by FIG. 1 in which the primary driver of each bullgear is a gas turbine and a steam turbine, respectively.

As a rule of thumb, in a combined cycle gas turbines system the maximal shaft power of a steam turbine is about 50% of the gas turbine. Due to the large number of impellers a discretization of the process is possible enabling at least one process cycle to be driven directly by the gas turbine, and at least one further to be split between the gas turbine bullgear and the steam turbine bullgear in order to approximate the 2:1 power ratio between the gas and steam turbines, respectively.

In FIG. 1 a process with three fluidly separated Brayton cycles are shown. A possible way of discretizing the process is indicated by FIG. 2 where the entire low-temperature Brayton cycle associated with expander 3 is connected to the gas turbine bull gear, the entire high temperature Brayton cycle including the expander 1 is connected to the steam turbine bull gear, and the intermediate temperature Brayton cycle comprising the expander 2 is split between the two bull gears.

In FIG. 2 a possible setup of the packages 200, 300 including a gas turbine 201 and steam turbine 301, respectively. The gas turbine package is including the low temperature expander 3, and the low temperature compressors 16, 17, 18 and medium temperature compressor 15 whereas the steam turbine package is comprising the medium and high temperature expanders 1, 2, and the medium temperature compressor 14 and the high temperature compressors 12, 13.

Such arrangement allows for starting up the low temperature loop independently of the other Brayton cycles assuming the intermediate temperature Brayton cycle compressor connected to the gas turbine bull gear is fully recycled. The main heat exchanger 8 or cold box can then be cooled down simultaneously as the steam generation is initiated. The mechanical lay out also allows for a coupling between the two driven integrally geared compressors via a clutch, shaft or gearbox, so that unbalance in the power delivered from the drivers and power demand from the integrally geared compressors can be handled.

The solution indicated above opens for the possibility of using the steam generated in the gas turbine directly in the steam turbine, at an optimum power generation and distribution and with a low specific power consumption. Normally the steam in a cogenerative setup is sent to a steam turbine, which drives a generator which in turn feeds an electric motor, with all the inherent energy losses. This system utilises the energy directly, and also makes it possible to make a mechanical lay out that is small in footprint, weight and cost. The steam is generated via coils in the exhaust stack in the gas turbine, and routed through the steam turbine at typically two pressure levels, one overheated high pressure steam level, and one medium pressure steam level.

The gas turbine has a waste heat recovery unit 202. The waste heat recovery unit is in principle a closed loop circulation consisting of a coil in the exhaust stack wherein the steam is generated by the excess heat in the exhaust. The steam therefrom this is directly utilized in the steam turbine. The two integrally geared compressor and expander setups may also be mechanically coupled via a clutch, shaft, or gear. A duct burner arranged in the waste heat recovery unit 202, not illustrated, may compensate for smaller deficits in steam demand whereby the process is optimized. As an alternative it is possible to use another suitable heat source not included in the heat recovery unit.

Each of the gas and steam turbines 201, 301 can mechanically be coupled to individual compressor sections having the expanders 1, 2, 3 arranged on separate stand-alone expander-booster-compressor skids, not shown in the drawings. 

1. A system for producing liquefied and sub-cooled natural gas by means of a refrigeration assembly using a single phase gaseous refrigerant comprising: at least two expanders; a compressor assembly; a heat exchanger assembly for heat absorption from natural gas; and a heat rejection assembly, in which the expanders are arranged in expander loops and the refrigerant to the respective expander is served in a compressed flow by the compressor assembly having compressors or compressor stages enabling adapted inlet and outlet pressures for the respective expander, wherein the expanders and compressors or their stages are assembled in two mechanically connected compressor and expander packages, of which one is driven by a gas turbine and the other is driven by a steam turbine, the steam primarily being generated by exhaust gases from the gas turbine in a waste heat recovery unit, and wherein the expanders and compressors or their stages are distributed between the two compressor and expander packages to optimize the steam utilization and to balance the power generated by the gas turbine and the steam turbine.
 2. A system according to claim 1, wherein a duct burner arranged in the waste heat recovery unit or another heat source compensates for smaller deficits in steam demand as to optimize the process.
 3. A system according to claim 1, wherein each of the gas and steam turbines are mechanically coupled to an integrated gear box comprising different combinations of compressor and expander units.
 4. A system according to claim 1, wherein each expander loop is part of fluidly separated refrigerant cycles, wherein all compressors and expanders are associated with at least one of the refrigerant cycles being arranged on the gearbox directly driven by the gas turbine so that cooling duty can be established and controlled independent of the status of the steam system.
 5. A system according to claim 1, wherein each of the gas and steam turbines are mechanically coupled to individual compressor sections, the expanders being arranged on separate stand-alone expander-booster-compressor skids. 